parallels: respect error code of bdrv_getlength() in allocate_clusters()

parallels: respect error code of bdrv_getlength() in allocate_clusters()

If we can not get the file length, the state of BDS is broken completely.
Return error to the caller.

Signed-off-by: Denis V. Lunev <den@openvz.org>
CC: Markus Armbruster <armbru@redhat.com>
CC: Kevin Wolf <kwolf@redhat.com>
CC: Max Reitz <mreitz@redhat.com>
CC: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>